Microwave antenna array



Oct. 13, 1959 H. SALTZMAN MICROWAVE ANTENNA ARRAY Filed April 50, 195'?" INVEN TOR. HENRY SALTZ MAN s LENGTH A. A. n /l! ATTORNEY.

2,908,905 MICROWAVE ANTENNA ARRAY Henry Saltzman, White Plains, N.Y., assignor toGeneral Precision Laboratory Incorporated, a corporation of New York Application April 30, 1957, Serial No. 658,212

r 6' Claims. or. 343-771 This invention relates to any transmission line network, and providesanimprovement in the matching of any discontinuity thereof. This invention is especially applicable to transmission lines for frequencies in the microwave region. I

As an illustration of the employment of this invention, its application to a resonant microwave linear array or radiators is selected. Aresonant linear array has its radi'tors regularly spaced at half-wave intervals. The radiators may be of any type such as, for example, seiies ors'hunt's'lots in rectangular waveguide. Such a resonant array is usually terminated in a short circuit. When the radiator discontinuities are not individually compensated such an array has a very narrow frequency transmission band.' .As a specific example, if the nominal frequency of theapplied microwave energy be ten kilomegacyclesper. second and departs by 30 mc. p. s. from this nominal value, 1'l.2% of the energy applied to such an array would'be reflected toward the source, corresponding to a voltage standing wave ratio (VSWR) of two to one on the source side of the array. This imposes such a seveire limitation on the constancy of frequency of the sourceasto make use of such an unmatched array diflicult'. f

"A reactive matching element such as a post or iris can beplaced on theinput sideof each radiator. This can makejthe linear array wider band by matching the impedance discontinuities but at the same time introduces phase .difiiculties.

,, When linear'j'arrays or planar arrays made therefrom are employed to generate a narrow beam or thin cone of mici owave radiation, the angle which the beam or c'one element makes with the longitudinal axis of the array isusually important. It is often required that this angle be precisely calculated in designing the array and.

that ,the measured beam angle of the constructed array be as calculated. It is. therefore importantthat when matching elements areused they do not disturb this beam When a single matching element is' associated with each radiator the beam angle is nearly always changed becauseeach matching element changes the phases of the exciting voltages applied to preceding radiators, andthe'resultis an undesirable beam angle change. This sitnation is made worse by the fact that illumination of the radiators by the applied energy is usually made to vary of the impedance discontinuity of each radiator be ef-' fected in two steps by two matching elements instead of one, not only is the impedance discontinuity 'matched just aspergfectly as by a single element but also the phases of 70 not afiected. The result is that the array frequency band the-exciting voltages applied to preceding radiators are iswidenedwithout disturbing the beam angle. A.,further 2,908,905. Patented Oct. 13, 1959 advantage is the removal of restrictions on the choice of the kind of illumination of the radiators.

As a specific example of this invention a rectangular waveguide linear array having shunt slot radiators at half-wavelength intervals is selected. It is to be under-. stood, however, that the application of this invention, is not confined to any particular type of linear array, to any particular kind of radiator, or to any particular spacing of radiators. This array is chosen because it is in general use and the invention is easily applicable to it..

. When such an array is terminated'in a short circuit and the individual radiators are unmatched, each radiator constitutesan admittance discontinuity and reflects part of the incident microwave energy back toward the source of energy. Since shunt slots may be designed to behave as pure shunt conductances, they are so assumed for purposes of discussion herein. However, this usual case is selected merelyfor simple illustration and the invention is applicable to any series or shunt radiators having any impedance or admittance phase angle.

Q It is to be understood that the character of any radiator or matching device is described with equal correctness as impedance or admittance, although in design it is usually more convenient to use one than the other. It. is

possible to choose the number of radiators and their conductance's so that the entire array will, at the design frealong the array in accordance with a chosen function to quency, present characteristic conductance to the source of microwave energy, will not reflect any energy to the source and will have a VSWR of unity at the source end.

Reflections and therefore standing waves will exist within modify it by terminating it in its characteristic admit-.

tance or in an absorbing device, and by applying a matching admittance at each radiator. tance is preferably purely susceptive and is positioned on the input side of the radiator-at a calculated distance.

The magnitude of the susceptance and its positioning can be so' chosen as exactly to match the conductance of the radiator. Such an array matches the input source and has a wider bandwidth than the previously-described ar- .ray. However, each matching susceptance changes the phase of the microwave voltage supplied to the preceding radiator from the phase which this voltage would have in the absence of the matching susceptances. The result is that the angle of the beam of microwave radiation emitted by the array may be modified. This result occursbecause. the beam angle depends not only on the microwave length in space and in the waveguide, and on the spacing s between radiators, all of which are constant, but also on the input voltage phase at each radi ator. Since this phase may be changed by the matching susceptances the beam angle may be changed.

When instead of one matching admittance preceding each radiator two are employed, with proper magnitudes and spacings, the effect is the same as described in the preceding single matching susceptance case except that the feed voltage phase at the preceding radiator is not disturbed and therefore the beam angle is not changed from its value in the unmatched array case. In a special case the two matching admittances are purely susceptive and when guide, the two susceptances must beof opposite signs.

Each matching admit- In general, however, the two admittances may be complexand may have the same or different signs.

The general purpose of this invention is to prov1de a microwave linear array containing two matching elements between each two radiators, these elements being so constructed and positioned as to prevent energy reflected t the source while at the same time providing broadband operation and having no effect on the beam angle of the design frequency.

Further understanding of this invention may be secured from the detailed description and associated drawings, in which;

Figure 1 depicts a rectangular microwave guide linear array embodying the invention.

Figure 2 is a portion of a Smith chart illustrating a method of designing the array of Fig. 1.

Figure 3 is a rectangular coordinate graph illustrating the operation of the array of Fig. 1.

Referring now to Fig. 1, a rectangular hollow Waveguide 11 is provided at one end with an absorbing termination 12 consisting of a carbon wedge. The other end 13 of the waveguide is designed to be connected to a source of microwave energy. Three shunt slots 14, 16 and 17 are positioned along one broad face 18 of the waveguide parallel to the center line 19 thereof. These slots are broadly resonant, being elfectively one-half wavelength long. They are spaced from the center line 19 by distances 0 determining the slot couplings. These couplings may be the same or different in accordance with the illumination function selected. The slots are spaced along the length of the waveguide at equal intervals s. Although only three slots are shown for simplicity, normally an antenna array has ten or more radiators and may have any number. The array also may be combined with other linear arrays to form a planar array.

The admittance of each slot is matched by two elements placed near it in the waveguide and between the slot and the power source. The first element is a septum 21 placed within the waveguide on the interior surface of the broad waveguide face opposite to the face pierced by the slots. This septum forms an iris opening at a distance d from the center of the succeeding radiator 16. Such an iris, if the septum be thin, constitutes a nearly pure capacitive susceptance. The second element is a septum 22 placed within the waveguide on one of the narrow faces. This septum, if thin, forms a nearly pure inductive susceptance iris opening at a distance d from the septum 21. The magnitudes of the susceptances B and B formed by irises 21 and 22 respectively are determined by their dimensions W and W Although it has been stated that the matching admittances are pure susceptances in the example, in the general case susceptances B and B need not be pure susceptances, but may instead be complex admittances Y and Y In this general case the four parameters Y Y d and d;, are interdependent and there is an infinitely large number of combinations which will match a given shunt slot and provide the described improved results.

The simple form of the invention described and depicted in Fig. 1 may be used as an example in describing the design method. Preferably a Smith chart is employed, such as described by P. H. Smith in Electronics for January 1939. In Fig. 2, representing a portion of such a chart, the point 23 represents on the vertical diametral scale the sum of the conductance G of slot 16, Fig. l, and of the line termination. For the present purpose it is sufficiently accurate to consider the slot shunt admittance as entirely conductive. The upper end 41, Fig. 2, of this vertical diametral scale is the zero conductance point of the scale. A circle 24 is now drawn through point 23 about the unit conductance point 26 as a center until it intersects the circle 27 which represents a. normalized conductance of unity. Progression clockwiseon circle 24 represents progression in waveguide 11,

4 Fig. 1, from the center of slot 16 toward the waveguide input end 13, and the amount of progression in fractions of is measured on a scale on the periphery 28, F1g. 2, by drawing the radius from center 26 through the point of intersection 29. The scale distance N: Fig. 2, then represents the waveguide distance d Fig.

1. A point 31 is now selected on circle 27' approximately half way between points 26 and 29. The distance 29-31 represents the susceptance B and its magnitude is ascertained on a second peripheral scale by subtracting the value at point 32 intersected by curve 32' from that at point 33 intersected by curve 33'. This matching susceptance B is positive, that is, it is capacitive. A circle 34 is now drawn through point 31 clockwise about center 26 until it meets the unity circle 27 at point 36. -Progression along circle 34 from point 31 to point 36 is measured by extending radii through points 31 and 36 to the first peripheral scale, and is termed M This scale distance represents the waveguide distance (1,, Fig. 1. The capacitive (positive) susceptance indicated by point 36, Fig. 2, is read from the second peripheral scale at the intersection 35 of the arc 35' through point 36. The lines 37, 38 and 39 are now drawn between the The fact that the are 3626 terminates at the center.

or match point of the Smith chart indicates that the admittance of the slot 16, Fig. 1, has been perfectly matched. That is, the reflected wave set up by the admittance discontinuity of the slot does not persist to the left of septum 22. Additionally, the fact that the angles a and b, Fig. 2, are equal indicates that the phase of the voltage applied to the slot 14 has not been disturbed from What it would be in an unobstructed line. This design procedure is to be followed for each slot and this analysis holds true :for all, therefore the composite beam angle of the entire linear array is not changed by the matching elements.

The described case is special in that both admittances 21 and 22, Fig. 1, are pure susceptances, this being demonstrated by the fact that the insertions of B and B causes the admittance to move along constant conductance circles of the Smith chart. 24 is not restricted to termination on the unit circle 27. If, for example, the are 24 should terminate before reaching the unit circle 27, say on the circle having value G=1.1, application of a general susceptance in place of iris 21 would be depicted, for example, by clockwise progression along the 1.1 circle toward the vertical diameter to some point. From this point a circle drawn clockwise about center 26 to the unit circle 27 would depict some distance such as '2 N1 and application of a pure inductive susceptance would move the operating location to the match point 26. This must be done choosing the proper angles at b.

The operation of this invention is further explained' by means of the rectangular coordinate graph of Fig. 3, Which applies to the linear array of Fig. 1. In this graph the abscissa scale constitutes waveguide longitudinal dis-. tance measured in units of A Progress along this ab- In the general case arc,

scissa scale fronts the'zleft toward the right represents distance measured along the waveguide from the input end toward the termination. The points S and S on the scale repiesent the" locations in- Fig. 1 of the slots 14 and 16 respectively. The ordinate scale represents the phase of the microwave voltage at any point in the waveguide relative to the voltage phaseat the same instant at location S1 of'slot I6.

Consider the efi'ect of microwave energy applied from the:v l@fit H1'l-the absence, of radiators and matching elements. The'guide is smooth, that is, it has no discontinuities, and: the smooth and continuous variation of phase is represented by the straight line 42, Fig. 3. Now consider the effect of adding radiators 14 and 16. spaced from each other a distance of one-half wavelength of the energy in the waveguide. Energy is reflected by slot 16 resulting in phase change on the input side of this slot. This is partially represented in Fig. 3 by the curve 43 between locations S and S In the absence of matching devices this curve would continue and cross the straight line 42, and then would meet it again at the location S of slot 14. Standing waves would exist to the left of location 8 Now suppose the capacitive matching element 21 is to be added. This is represented in Fig. 3 at location S by a Vertical line 44, and produces a change in the rate of curvature of the line representing phase, with the line 46 now representing phase between locations S and S At the latter location the line 46 becomes tangent to the straight line 42. A standing wave exists to this point.

At location S suppose the second matching element 22 be applied. This matches the mismatch existing at location S in such a manner that between locations S and S the phase is represented by the straight line 42. Between these locations there is no standing wave. That is, there is no reflected wave between iris 22 and slot 14. The fact that the line passes through point 47, Fig. 3, means that the phase at this point is the same as the phase of an unobstructed line would be and the same as the phase would be at the point in the case of unmatched radiators N1 2 distance apart.

Referring again to Fig. 1, the coupling distances C will in general be different for the different radiators, resulting in different admittances for the dilferent radiators. This in turn will require different matching admittances, each pair and its distances being calculated for a specific radiator and different from each other pair and its placement distances.

What is claimed is:

. 1. A wideband microwave linear array comprising, a plurality of radiators each constituting an impedance discontinuity, each radiator being preceded at a selected distance by a first matching element having a first selected impedance, each of said first matching elements being preceded at a second selected distance by a second matching element having a second selected impedance, each of said first and second matching elements shifting the phase of the microwave energy existing in said linear array, said first and second impedance acting together to neutralize said impedance discontinuity while at the same time mutually cancelling the phase shifting effect of each alone.

2. A wideband microwave linear array comprising, a hollow waveguide section having a source of microwave energy coupled to one end thereof, a plurality of radiating elements equally spaced along the linear length of said waveguide section each coupled to the energy contained therein and interrupting the impedance continuity thereof, a plurality of matching elements equal in number to the number of radiating elements each having a selected individual impedance and causing a selected: phase shift of. said energy, said matchingelements;

being-positioned internally of said waveguide section with respective ones thereof spaced from respective ones ofsaid radiating elements at selected distances in the direction towards said source of energy, and a plurality of other matching elements equal in number to the number of radiating. elements each having a selected indipedance of one of said other matching elements to neutralize the impedance interruption of one of said radiating elements, and the phase shift of each one of said matching elements being of such magnitude in combination with the phase shift of one of said other matching elements forming a pair, that no phase disturbance is caused in said energy between the source thereof and the nearer one of said pair.

3. A wideband microwave linear array comprising, a hollow waveguide section having a source of microwave energy coupled to one end thereof, a plurality of radiating elements equally spaced at distances of one-half wavelength of said energy in waveguide along the length of said section and coupled to the energy therein, the magnitude of coupling being different for different radiating elements whereby the impedance which each presents to said energy is a function of the coupling, first individual matching impeders for each of said radiating elements each positioned in said waveguide at a selected distance preceding the associated radiating element, the impedance and position of each impeder being selected partly to neutralize the impedance mismatch and phase shift of its associated radiating element, second individual matching impeders for each of said radiating elements each positioned in said waveguide at a selected distance preceding individual ones of said first matching impeders, the signs of said first and second matching impeders being different with the impedance of each individual one of said second matching impeders and its positioned distance being selected completely to neutralize the impedance mismatch and phase shift of its associated individual radiating element left unneutralized by the associated one of said first matching impeders.

4. A wideband microwave linear array comprising, a rectangular waveguide section provided with a linear array of slot radiators regularly spaced along the length thereof, a source energizing said Waveguide, and a pair of impedance and phase shift matching elements positioned between each pair of radiating elements, each pair of matching elements consisting of a capacitive element and an inductive element affixed to said waveguide section at such selected distances preceding individual slot radiators as to neutralize the impedance discontinuity and phase shift presented by each slot radiator to said energy source.

5. A wideband microwave linear array comprising, a rectangular waveguide section provided with a linear array of slot radiators regularly spaced along the length thereof, a source energizing said linear array, '2. pair of impedance discontinuity and phase shift correcting elements positioned between each pair of radiating elements, each pair of correcting elements consisting of a capacitive element afiixed to said waveguide section at a selected distance preceding an individual slot radiator and an inductive element aifixed to said waveguide section at a selected distance preceding its associated capacitive element, said individual capacitive and inductive elements mutually cooperating to neutralize the impedance discontinuity and the phase shift presented to said energy source by the slot radiator they precede.

6. A wideband microwave linear array comprising, a

7. rectangular waveguide section provided with a linear array of slot radiators regularly spaced along the length thereof, a microwave source energizing said linear array, a pair of impedance discontinuity and phase shift correcting elements positioned between each pair of radiating elements, each pair of correcting elements consisting of a capacitive iris positioned internally of said wavc guide section at a selected distance preceding an individual slot radiator and an inductive iris positioned internally of said waveguide section at aselected distance preceding its associated capacitive iris, each of said pair of irises mutually cooperating to neutralize the impedance discontinuity and the phase shift presented to said Broad-Band Wave-Guide Admittance Matching by Use of Irises, by R. G. Fellers and R. T. Weidner, Proceedings of the Institute of Radio Engineers, vol. 35, No, 10, October 1947, pp. 1080-1085. 

